Update forking_workflow.md

Adding a recommendation to use Repository mirroring

doc/user/project/repository/forking_workflow.md

@@ -41,6 +41,10 @@ CAUTION: **CAUTION:**
 From GitLab 12.6 onwards, if the [visibility of an upstream project is reduced](../../../public_access/public_access.md#reducing-visibility)
 in any way, the fork relationship with all its forks will be removed.
 
+CAUTION: **Caution:**
+[Repository mirroring](repository_mirroring.md) will help to keep your fork synced with the original repository.
+Before approving a merge request you'll likely to be asked to sync before getting approval, hence automating it is recommend.
+
 ## Merging upstream
 
 Once you are ready to send your code back to the main project, you need